1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus.
2. Description of the Related Art
In the commercial printing industry, in order to print various kinds of variable data of small lots, the conventional offset printing is shifting to POD (Print on Demand) using an image forming apparatus of an electrophotographic scheme. In the image forming apparatus using the electrophotographic scheme, accuracy of front-to-back registration equivalent to the offset printer is being required in order to meet the needs of POD.
Factors of front-to-back misregistration can be largely classified to a registration error in vertical and lateral directions, and a skew error of sheet/image. For an image forming apparatus having a heat fixing device, an image scaling error due to expansion and contraction of the sheet is added as a factor.
In order to automatically correct the image scaling error between the front and the back of the sheet, it is necessary to accurately measure a sheet size and a distance by which the sheet is conveyed. Various techniques have been proposed to achieve this objective. But, there is a problem in that measurement accuracy deteriorates due to inadequate detection accuracy of a sheet.